Catalyst systems comprising a metallocene complex of a transition metal, e.g., Ti, Zr or Hf, and an organoaluminum-oxy compound, e.g., methylaluminoxane, have been attracting attention as a catalyst for olefin polymerization. They have high activity particularly in copolymerization of ethylene and an .alpha.-olefin to provide excellent polymers having narrow molecular weight distribution and narrow composition distribution. These catalysts are described in detail in, e.g., J.M.S.--Rev. Macromol. Chem. Phys., C34, No. 3, p. 439 (1994).
Catalyst systems containing a compound of a transition metal, e.g., Ti, V, Co, Ni or Nd, are known for catalysis in coordination anionic polymerization of conjugated dienes, such as butadiene and isoprene. For example, a method of using VCl.sub.3 --AlR.sub.3 as a catalyst of conjugated diene polymerization is reported, e.g., in Chim. e Ind., Vol. 40, p. 362 (1958). The result as reported, however, is production of crystalline polybutadiene having nearly 100% of a trans structure, and the catalyst has extremely low polymerization activity.
International Rubber Conference Kobe, Preprint, 25C-4 (1995) reports that V(acac).sub.3 -methylaluminoxane provides polybutadiene comprising a cis structure, a trans structure, and a 1,2-structure in a proportion of 63%, 14%, and 21%, respectively, but the catalyst activity is still low.
Polymerization of conjugated diene by using a metallocene catalyst of the group IV transition metal (e.g., Ti or Zr) is reported in Macromolecular Symposia, Vol. 89, p. 383 (1995), in which a catalyst system containing cyclopentadienyltitanium trichloride [(.eta..sup.5 C.sub.5 H.sub.5)TiCl.sub.3 ] is used. The activity of this catalyst system in conjugated diene polymerization is not deemed sufficient because the polymerization activity of (.eta..sup.5 C.sub.5 H.sub.5)TiCl.sub.3 -methylaluminoxane, for instance, is, as reported, 100 g/mmol-Ti/hr at the most.
Macromolecular Symposia, Vol. 4, p. 103 (1986) reports copolymerization of ethylene and butadiene in the presence of a combination of bis(cyclopentadienyl)zirconium dichloride [(.eta..sup.5 C.sub.5 H.sub.5).sub.2 ZrCl.sub.2 ] and methylaluminoxane, but the report has no mention of the comonomer composition of the resulting polymer. Neither does the report refer to homopolymerization of butadiene. The reported catalyst activity is very low.
With regards to a polymerization catalyst comprising a metallocene compound of vanadium (V), the group V transition metal, JP-64-66216 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") (U.S. Pat. No. 5,204,429), JP-W-63-501962 (the term "JP-W" as used herein means an "unexamined published international patent application") (U.S. Pat. Nos. 4,752,597 and 4,791,180), and JP-W-1-501633 (U.S. Pat. Nos. 5,191,052, 4,808,561 and 4,897,455) disclose a combination of a metallocene compound of vanadium and an aluminoxane as a catalyst for copolymerization of an olefin and a diene. These publications refer to bis(cyclopentadienyl)vanadium dichloride [(.eta..sup.5 C.sub.5 H.sub.5).sub.2 VCl.sub.2 ] as an illustrative example of the vanadium metallocene compounds without giving any working example using this particular compound.
JP-B-46-20494 (the term "JP-B" as used herein means an "examined published Japanese patent application") discloses a process for producing polybutadiene using a catalyst system comprising a cyclopentadienyl complex of vanadium, a halogen-containing organoaluminum compound, an oxygen-containing compound. (C.sub.5 H.sub.5)VCl.sub.3 is mentioned as an example of the cyclopentadienyl complex of vanadium, but its polymerization activity is problematically low. According to the disclosure, it is essential for the organoaluminum compound, which serves as a co-catalyst, to contain a halogen component. For example, a working example using (C.sub.5 H.sub.5)VCl.sub.3 --Al(i-Bu).sub.3 /AlCl.sub.3 is given, but the polymerization activity attained in that example is 350 g/mmol-V/hr at the most.
Polymer, Vol. 37 (2), p. 363 (1966) describes a process for producing polybutadiene, in which a catalyst comprising a vanadium (III) compound, e.g., (substituted C.sub.5 H.sub.5)VCl.sub.2 .cndot.(PR.sub.3).sub.2 or (substituted C.sub.5 H.sub.5).sub.2 VCl, and methylaluminoxane is used to obtain polybutadiene of high cis structure having a 1,2-structure content of 10 to 20%. The polymerization activity of the catalyst is low however.
It is known that conjugated diene polymers take various microstructure depending on the polymerization catalyst used. While polybutadiene, for instance, is generally prepared by polymerization of 1,3-butadiene, polybutadiene as produced comprises different microstructures in its molecular chain, i.e., a structural unit produced through 1,4-addition (1,4-structure) and a structural unit produced through 1,2-addition (1,2-structure), due to the difference in mode of addition. The 1,4-structure is further divided into a cis-1,4-structure and a trans-1,4-structure. The 1,2-structure has a vinyl group as a side chain and can have an isotactic structure, an syndiotactic structure, and an atactic structure. The above-described macrostructure varies depending on the catalyst of polymerization, and polybutadiene species having different microstructures find their respective uses according to their characteristics. In particular, polybutadiene mainly comprising a cis structure and a moderate proportion of a 1,2-structure is expected as an impact modifier for plastics such as polystyrene.
Polybutadiene produced by using a butyllithium catalyst is known for use in high impact polystyrene (HIPS). Since it has a smaller content of a cis structure than the one produced by using a cobalt catalyst, an improvement in viscosity and the like has been demanded in the production of HIPS.
Cobalt catalyst systems that provide polybutadiene containing both a cis structure and a 1,2-structure include a catalyst system comprising a phosphate, an organoaluminum compound, water, and a cobalt compound and a catalyst system comprising an organoaluminum compound, water, and a cobalt dithiocarbamate compound as disclosed in JP-A-55-129403 and JP-A-59-232106. These catalysts sometimes provide polybutadiene having a low molecular weight, or they are not deemed to have sufficient catalyst activity.
JP-B-62-16201 discloses a process of using a catalyst system comprising a cobalt salt, a dialkylaluminum monohalide, and a dihydric alcohol. According to the disclosure, the resulting polybutadiene comprises 4 to 20% of a 1,2-structure, 75 to 95% of a cis-1,4-structure, and not more than 4% of a trans-1,4-structure, in which the content of 1,2-structures distributed at random is higher than the content of those connected to each other to form blocks, and is suitable for the production of HIPS.
For use as an impact modifier, polybutadiene must have a controlled molecular weight. For example, in the preparation of polybutadiene having a high cis content by using the above-described cobalt compound-organoaluminum compound catalyst system, it has been proposed to add a non-conjugated diene compound, such as cyclooctadiene, to the polymerization system as disclosed in JP-B-35-495.
However, addition of cyclooctadiene to the polymerization system comprising a metallocene complex of the group V transition metal unsuccessfully produces a sufficient effect for molecular weight reduction, only to lead to reduced polymerization activity.
JP-B-52-32912 (U.S. Pat. No. 3,983,183) and JP-B-56-50894 (U.S. Pat. No. 3,966,697) disclose a process for producing polybutadiene having a high 1,2-structure content by using a specific catalyst system. The Examples of the disclosures reveal that an increase in molecular weight of polybutadiene results in an increase in gel content.
A polybutadiene production system is liable to suffer from gelation in the polymer or the polymerization vessel. Gelation becomes conspicuous as the molecular weight increases. It is important for high-molecular weight polybutadiene, which is generally used as a mixture with an oil extender or a low-molecular polymer, to have an extremely low gel content formed during polymerization.
JP-B-3-57128 shows a process for producing polybutadiene having a cis-1,4-structure content of 50% or higher, a 1,2-structure content of 7 to 50%, an intrinsic viscosity of not less than 1, and a gel content of not more than 0.03%, which comprises using a catalyst system comprising a halogenated organoaluminum compound, a cobalt dithiocarbamate compound, water, and a free radical scavenger. However, the intrinsic viscosity of the polybutadiene obtained in the Examples is 2.5 at the most.